The Lower Permian Wasp Head Formation,Sydney Basin: high‐latitude,shallow marine sedimentation following the late Asselian to early Sakmarian glacial event in eastern Australia

The Lower Permian Wasp Head Formation (early to middle Sakmarian) is a ～95 m thick unit that was deposited during the transition to a non‐glacial period following the late Asselian to early Sakmarian glacial event in eastern Australia. This shallow marine, sandstone‐dominated unit can be subdivided into six facies associations. (i) The marine sediment gravity flow facies association consists of breccias and conglomerates deposited in upper shoreface water depths. (ii) Upper shoreface deposits consist of cross‐stratified, conglomeratic sandstones with an impoverished expression of the Skolithos Ichnofacies. (iii) Middle shoreface deposits consist of hummocky cross‐stratified sandstones with a trace fossil assemblage that represents the Skolithos Ichnofacies. (iv) Lower shoreface deposits are similar to middle shoreface deposits, but contain more pervasive bioturbation and a distal expression of the Skolithos Ichnofacies to a proximal expression of the Cruziana Ichnofacies. (v) Delta‐influenced, lower shoreface‐offshore transition deposits are distinguished by sparsely bioturbated carbonaceous mudstone drapes within a variety of shoreface and offshore deposits. Trace fossil assemblages represent distal expressions of the Skolithos Ichnofacies to stressed, proximal expressions of the Cruziana Ichnofacies. Impoverished trace fossil assemblages record variable and episodic environmental stresses possibly caused by fluctuations in sedimentation rates, substrate consistencies, salinity, oxygen levels, turbidity and other physio‐chemical stresses characteristic of deltaic conditions. (vi) The offshore transition‐offshore facies association consists of mudstone and admixed sandstone and mudstone with pervasive bioturbation and an archetypal to distal expression of the Cruziana Ichnofacies. The lowermost ～50 m of the formation consists of a single deepening upward cycle formed as the basin transitioned from glacioisostatic rebound following the Asselian to early Sakmarian glacial to a regime dominated by regional extensional subsidence without significant glacial influence. The upper ～45 m of the formation can be subdivided into three shallowing upward cycles (parasequences) that formed in the aftermath of rapid, possibly glacioeustatic, rises in relative sea‐level or due to autocyclic progradation patterns. The shift to a parasequence‐dominated architecture and progressive decrease in ice‐rafted debris upwards through the succession records the release from glacioisostatic rebound and amelioration of climate that accompanied the transition to broadly non‐glacial conditions.     

Late Quaternary submarine bedforms and ice‐sheet flow in Gerlache Strait and on the adjacent continental shelf,Antarctic Peninsula

Geophysical data from Gerlache Strait, Croker Passage, Bismarck Strait and the adjacent continental shelf reveal streamlined subglacial bedforms that were produced at the bed of the Antarctic Peninsula Ice Sheet (APIS) during the last glaciation. The spatial arrangement and orientation of these bedforms record the former drainage pattern and flow dynamics of an APIS outlet up‐flow, and feeding into, a palaeo‐ice stream in the Western Bransfield Basin. Evidence suggests that together, they represent a single ice‐flow system that drained the APIS during the last glaciation. The ice‐sheet outlet flowed north/northeastwards through Gerlache Strait and Croker Passage and converged with a second, more easterly ice‐flow tributary on the middle shelf to form the main palaeo‐ice stream. The dominance of drumlins with low elongation ratios suggests that ice‐sheet outlet draining through Gerlache Strait was comparatively slower than the main palaeo‐ice stream in the Western Bransfield Basin, although the low elongation ratios may also partly reflect the lack of sediment. Progressive elongation of drumlins further down‐flow indicates that the ice sheet accelerated through Croker Passage and the western tributary trough, and fed into the main zone of streaming flow in the Western Bransfield Basin. Topography would have exerted a strong control on the development of the palaeo‐ice stream system but subglacial geology may also have been significant given the transition from crystalline bedrock to sedimentary strata on the inner–mid‐shelf. In the broader context, the APIS was drained by a number of major fast‐flowing outlets through cross‐shelf troughs to the outer continental shelf during the last glaciation. Copyright © 2004 John Wiley & Sons, Ltd.     

The development of near-surface ground ice during the Holocene at sites near Mayo,Yukon Territory,Canada

An examination of soil thermal and hydrologic regimes at several sites near Mayo, Yukon Territory, and the ground-ice stratigraphy of a thaw-slump headwall close to these sites supports the hypothesis that an ice-rich zone, frequently observed at the base of the active layer, may be due to the annual water balance of permafrost. Observations demonstrate that ice lenses have grown at the top of permafrost in soil tubes installed in 1983. In addition, 90 cm of segregated ice are exposed in the thaw-slump headwall above a thaw unconformity, dated at 8870 ± 200 years BP, but below the base of the present active layer. These data suggest that the rate of water incorporation into permafrost over the last 8000 years in the Mayo area has been at least 0.1 mm yr^?1.     

The Donegal ice dome,northwest Ireland: dimensions and chronology

Geomorphological evidence indicates that Donegal was formerly occupied by an ice dome that extended offshore to the west, northwest and north and was confluent with adjacent ice masses to the east and south. Erosive warm‐based ice over‐rode almost all the highest mountains, implying an ice‐divide altitude greater than 700 m. Only six peripheral summits escaped glacial modification, implying either that they remained above the ice surface as nunataks or supported a thin cover of protective cold‐based ice. Gibbsite, a pre‐last glacial weathering product, is preferentially represented on summits that escaped glacial modification. Cosmogenic ¹⁰Be exposure ages of 18.6 ± 1.4 to 15.9 ± 1.0 k yr for coastal sites confirm that Donegal ice extended offshore at the last glacial maximum. Reconstruction of the form of the Donegal ice dome suggests a former minimum ice thickness of ～500 m close to the present coastline in the west and northwest, and ～400 m near the coast of the Inishowen Peninsula in the north, with the ice extending at least 20 km across the adjacent shelf to the west and northwest. Copyright © 2007 John Wiley & Sons, Ltd.     

Experimental soft‐sediment deformation caused by fluidization and intrusive ice melt in sand

Identifying the driving mechanisms of soft‐sediment deformation in the geological record is the subject of debate. Thawing of ice‐rich clayey silt above permafrost was proved experimentally to be among the processes capable of triggering deformation. However, previous work has failed so far to reproduce similar structures in sand. This study investigates fluidization and intrusive ice formation from soil models in the laboratory. Experimental conditions reproduce the growth of ice‐cored mounds caused by pore water pressure increase during freeze‐back of sand in a permafrost context. Excess pore water pressure causes hydraulic fracturing and the development of water lenses beneath the freezing front. Later freezing of the water lenses generates intrusive ice. The main structures consist of sand dykes and sills formed when the increase in pore water pressure exceeds a critical threshold, and soft‐sediment deformations induced by subsidence during ice melt. The combination of processes has resulted in diapir‐like structures. The experimental structures are similar to those described in Pleistocene sites from France. These processes constitute a credible alternative to the seismic hypothesis evoked to explain soft‐sediment deformation structures in other European regions subjected to Pleistocene cold climates.     

Evidence for a mid‐Pleistocene shift of ice‐drift pattern in the Nordic seas

Sediment proxy records from a continuous, 1.5 million year long deep‐sea sediment core from a site in the western Norwegian Sea were used to obtain new insights into the nature of palaeoceanographic change in the northern North Atlantic (Nordic seas) during the climatic shift of the Mid‐Pleistocene Revolution (MPR). Red‐green sediment colour and magnetic susceptibility records both reveal significant differences in their mean values when comparing the intervals older than 700 000 yr (700 ka) with those from the past 500 kyr. The timing and duration of these changes indicates that the MPR in the Nordic seas is characterised by a gradual transition lasting about 200 kyr. Together with further sedimentological evidence this suggests that the mid‐Pleistocene climate shift was accompanied by a general change in ice‐drift pattern. It is further proposed that prior to the onset of the major late Pleistocene glaciations in the Northern Hemisphere a significant proportion of the ice in the eastern Nordic seas originated from a southern provenance, whereas later it dominantly came from the surrounding landmasses. Copyright © 2003 John Wiley & Sons, Ltd.     

Ice wedges: Growth,thaw transformation,and palaeoenvironmental significance

Frost-cracking and ice-wedge growth are fundamental processes within the permafrost environment. Extensive areas of contemporary permafrost terrain are characterised by frost-fissure polygons, formed by repeated thermal contraction-cracking of the ground. The incremental growth of ice veins and wedges along the axes of contraction-cracks contributes significantly to the volume of ground ice in near-surface permafrost. In areas beyond the present limit of permafrost, the recognition of ice-wedge pseudomorphs provides one of the few unambiguous indications of the former existence of permafrost conditions. An understanding of the processes of ice-wedge growth and thaw transformation is essential if contemporary ice wedges are to be used as analogues for Pleistocene frost-fissure structures, in palaeoenvironmental reconstructions.     

东南极洲中山站附近1年生海冰的发育和结构特征及其对生物群落的影响(英)

Sea ice growth, characteristics and ice algal biomass were measured at four sites off Zhongshan Station (66°22, 24" S, 76°22, 40" E), East Antarctica in 1992. The ice formed in late March,grew stablely and reached its highest thickness of 174 cm in November and December. The difference　between measured ice thickness and the calculated values from the classical Stephen formula are small,suggesting that the heat flux from the ocean to the underside of the ice at this area is not obvious.The results of sea ice analysis show that the sea ice consists mainly of congelation ice, and the frazil　ice only occurs at the top of sea ice, no snow or platelet ice is found. When the ice thickness reached its　　highest values in late spring, congelation ice contained average 89. 5 % of the total column crystal 　structure, the remaining 10. 5% consisted of frazil ice, and the ice thicknesses above the congelation ice layers were mostly consistent throughout the year, suggesting that the sea ice developed in a relativelycalm environment after the top layer of ice had formed and grew downwards only.Congelation ice fabrics shows frequent moderate to strong alignments at almost all depths, and thedegree of preferred orientation mostly increased with depths. In the sub--structure of congelation ice,the plate width are in a range of 0. 55 to 1. 05 mm with an average value of 0. 76 mm. No apparent pattern occurs in its vertical profiles.Ice crystal composition is one of the major factors influencing the ice algal biomass, and the effectsof crystal alignments and ice plate width of congelation ice are not apparent.     

Late Holocene environmental conditions in Coronation Gulf,southwestern Canadian Arctic Archipelago: evidence from dinoflagellate cysts,other non‐pollen palynomorphs,and pollen

Boxcore 99LSSL‐001 (68.095° N, 114.186° W; 211 m water depth) from Coronation Gulf represents the first decadal‐scale marine palynology and late Holocene sediment record for the southwestern part of the Northwest Passage. The record was studied for organic‐walled microfossils (dinoflagellate cysts, non‐pollen palynomorphs), pollen, terrestrial spores, and sediment characteristics. ²¹⁰Pb, ¹³⁷Cs, and three accelerator mass spectrometry ¹⁴C dates constrain the chronology. Three prominent palaeoenvironmental zones were identified. During the interval AD 1470–1680 (Zone I), the climate was warmer and wetter than at present, and environmental conditions were more favourable to biological activity and northward boreal forest migration, with reduced sea‐ice and a longer open‐water (growing) season. The interval AD 1680–1940 (Zone II) records sea‐ice increase, and generally cool, polar conditions during the Little Ice Age. During AD 1940–2000 (Zone III), organic microfossils indicate an extended open‐water season and decreased sea‐ice, with suggested amelioration surpassing that of Zone I. Although more marine studies are needed to place this record into an appropriate context, the succession from ameliorated (Zone I) to cooler, sea‐ice influenced conditions (Zone II) and finally to 20th‐century warming (Zone III) corresponds well with several terrestrial climatic records from the neighbouring mainland and Victoria Island, and with lower‐resolution marine records to the west. Copyright © 2011 John Wiley & Sons, Ltd.     

Depositional history of the North Taymyr ice‐marginal zone,Siberia—a landsystem approach

The sediment–landform associations of the northern Taymyr Peninsula in Arctic Siberia tell a tale of ice sheets advancing from the Kara Sea shelf and inundating the peninsula, probably three times during the Weichselian. In each case the ice sheet had a margin frozen to its bed and an interior moving over a deforming bed. The North Taymyr ice‐marginal zone (NTZ) comprises ice‐marginal and supraglacial landsystems dominated by thrust‐block moraines 2–3 km wide and large‐scale deformation of sediments and ice. Large areas are still underlain by remnant glacier ice and a supraglacial landscape with numerous ice‐walled lakes and kames is forming even today. The proglacial landsystem is characterised by subaqueous (e.g. deltas) or terrestrial (e.g. sandar) environments, depending on location/altitude and time of formation. Dating results (OSL, ¹⁴C) indicate that the NTZ was initiated ca. 80 kyr BP during the retreat of the Early Weichselian ice sheet and that it records the maximum limit of a Middle Weichselian glaciation (ca. 65 kyr BP). During both these events, proglacial lakes were dammed by the ice sheets. Part of the NTZ was occupied by a thin Late Weichselian ice sheet (20–12 kyr BP), resulting in subaerial proglacial drainage. Copyright © 2002 John Wiley & Sons, Ltd.     

Seeking a Holocene drift ice proxy: non‐clay mineral variations from the SW to N‐central Iceland shelf: trends,regime shifts,and periodicities

Quantitative X‐ray diffraction analysis of the <2 mm sediment fraction was carried out on 1257 samples (from the seafloor and 16 cores) from the Iceland shelf west of 18° W. All but one core (B997‐347PC) were from transects along troughs on the NW to N‐central shelf, an area that in modern and historic times has been affected by drift ice. The paper focuses on the non‐clay mineralogy of the sediments (excluding calcite and volcanic glass). Quartz and potassium feldspars occupy similar positions in an R‐mode principal component analysis, and oligoclase feldspar tracks quartz; these minerals are used as a proxy for ice‐rafted detritus (IRD). Accordingly, the sum of these largely foreign minerals (Q&K) (to Icelandic bedrock) is used as a proxy for drift ice. A stacked, equi‐spaced 100 a record is developed which shows both low‐frequency trends and higher‐frequency events. The detrended stacked record compares well with the flux of quartz (mg cm^?2 a^?1) at MD99‐2269 off N Iceland. The multi‐taper method indicated that there are three significant frequencies at the 95% confidence level with periods of ca. 2500, 445 and 304 a. Regime shift analysis pinpoints intervals when there was a statistically significant shift in the average Q&K weight %, and identifies four IRD‐rich events separated by intervals with lower inputs. There is some association between peaks of IRD input, less dense surface waters (from δ¹⁸O data on planktonic foraminifera) and intervals of moraine building. Copyright © 2009 John Wiley & Sons, Ltd.     

Periglacial trimlines and nunataks of the Last Glacial Maximum: the Gap of Dunloe,southwest Ireland

During the last main phase of glaciation (26–13 ka) an ice‐cap developed in southwest Ireland and ice, from a dispersal centre in the vicinity of Kenmare, flowed north through the Gap of Dunloe in the Macgillycuddy's Reeks. On surrounding hillsides a weathering limit separates ice‐moulded bedrock, on low ground, from frost‐weathered terrain above. Assessment of bedrock dilation joint characteristics, Schmidt hammer R‐value data and clay‐sized mineral contents of basal soil samples, demonstrate significant contrasts in the degree of weathering above and below this limit. The weathering limit declines in altitude along former ice flow‐lines and is confluent with morainic deposits on the eastern side of the Gap. This supports the assertion that the high‐level weathering limit is a periglacial trimline that marks the former maximum upper limit of the body of ice which occupied the Gap of Dunloe during the Last Glacial Maximum (LGM). Reconstruction of the former ice‐surface profile from periglacial trimline limits on the eastern side of the Gap yields a mean estimate for basal shear stress of 106.5 kPa. This value suggests that the ice mass which occupied the Gap of Dunloe at the LGM was warm based and flowed on a bedrock substrate. Copyright © 2004 John Wiley & Sons, Ltd.     

Oxygen isotope and palaeotemperature records from six Greenland ice‐core stations: Camp Century,Dye‐3, GRIP,GISP2, Renland and NorthGRIP

Oxygen isotope variations spanning the last glacial cycle and the Holocene derived from ice‐core records for six sites in Greenland (Camp Century, Dye‐3, GRIP, GISP2, Renland and NorthGRIP) show strong similarities. This suggests that the dominant influence on oxygen isotope variations reflected in the ice‐sheet records was regional climatic change. Differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios. Palaeotemperature estimates have been obtained from the records using three approaches: (i) inferences based on the measured relationship between mean annual δ¹⁸O of snow and of mean annual surface temperature over Greenland; (ii) modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile, or (iii) by using Monte Carlo simulation techniques. The third of these approaches was adopted to reconstruct Holocene temperature variations for the Dye 3 and GRIP temperature profiles, which yields remarkably compatible results. A new record of Holocene isotope variations obtained from the NorthGRIP ice‐core matches the GRIP short‐term isotope record, and also shows similar long‐term trends to the Dye‐3 and GRIP inverted temperature data. The NorthGRIP isotope record reflects: (i) a generally stronger isotopic signal than is found in the GRIP record; (ii) several short‐lived temperature fluctuations during the first 1500 yr of the Holocene; (iii) a marked cold event at ca. 8.2 ka (the ‘8.2 ka event’); (iv) optimum temperatures for the Holocene between ca. 8.6 and 4.3 ka, a signal that is 0.6‰ stronger than for the GRIP profile; (v) a clear signal for the Little Ice Age; and (vi) a clear signal of climate warming during the last century. These data suggest that the NorthGRIP stable isotope record responded in a sensitive manner to temperature fluctuations during the Holocene. Copyright © 2001 John Wiley & Sons, Ltd.     

基于浅层探冰雷达的东南极内陆地区冰盖内部等时层提取

中国第32次南极考察内陆队首次使用雪地车载浅层探冰雷达取得了中山站至昆仑站全程1 300 km 的浅层冰盖数据,通过MATLAB对数据进行FFT转换、背景滤波、改变叠加次数去噪和基于曲波变换的图像处理,使用Reflexw对MATLAB处理后数据进行地形校正、二次背景滤波和平滑滤波等处理,成功提取了冰盖浅层等时层并得出了等时层的平均深度、最大深度和波动情况。最后进行了数据的连接,展示了LGB69冰芯处等时层的分布并与冰芯数据结合大体对应了相应等时层的年代和事件。给出了Dome A附近的等时层分布情况。为东南极中山站至昆仑站沿途的物质积累率计算提供重要的数据支撑。     

Geological evidence for neotectonic activity during deglaciation of the southern Sperrin Mountains,Northern Ireland

In the southern Sperrin Mountains, Northern Ireland, stacked glacigenic sequences that accumulated during deglaciation (ca. 17000–13000 yr BP) overlie a basement of jointed and mascerated bedrock. The glacigenic sequences comprise interbedded glaciotectonic shears, diamictic breccias and rock rafts. At one site a normal fault with a metre‐scale vertical displacement cuts through part of the sequence and is overlain by a glacial diamict. Sediments at an adjacent site show that faulting and associated hydrothermal activity was related to neotectonic reactivation of pre‐existing Caledonian lineaments caused by ice unloading. From stratigraphical and directional evidence, fault reactivation occurred early in the deglaciation after north central Ireland ice had retreated southwards into lowland areas, but before Sperrin Mountain ice readvanced from the north. This relationship provides evidence for the relative timing of neotectonic activity in Northern Ireland, and demonstrates the effects of glacio‐isostatic unloading near ice‐sheet centres. Copyright © 1999 John Wiley & Sons, Ltd.     

Marine geophysical evidence for former expansion and flow of the Greenland Ice Sheet across the north‐east Greenland continental shelf

High‐resolution swath bathymetry and TOPAS sub‐bottom profiler acoustic data from the inner and middle continental shelf of north‐east Greenland record the presence of streamlined mega‐scale glacial lineations and other subglacial landforms that are formed in the surface of a continuous soft sediment layer. The best‐developed lineations are found in Westwind Trough, a bathymetric trough connecting Nioghalvfjerdsfjorden Gletscher and Zachariae Isstrøm to the continental shelf edge. The geomorphological and stratigraphical data indicate that the Greenland Ice Sheet covered the inner‐middle shelf in north‐east Greenland during the most recent ice advance of the Late Weichselian glaciation. Earlier sedimentological and chronological studies indicated that the last major delivery of glacigenic sediment to the shelf and Fram Strait was prior to the Holocene during Marine Isotope Stage 2, supporting our assertion that the subglacial landforms and ice sheet expansion in north‐east Greenland occurred during the Late Weichselian. Glacimarine sediment gravity flow deposits found on the north‐east Greenland continental slope imply that the ice sheet extended beyond the middle continental shelf, and supplied subglacial sediment direct to the shelf edge with subsequent remobilisation downslope. These marine geophysical data indicate that the flow of the Late Weichselian Greenland Ice Sheet through Westwind Trough was in the form of a fast‐flowing palaeo‐ice stream, and that it provides the first direct geomorphological evidence for the former presence of ice streams on the Greenland continental shelf. The presence of streamlined subglacially derived landforms and till layers on the shallow AWI Bank and Northwind Shoal indicates that ice sheet flow was not only channelled through the cross‐shelf bathymetric troughs but also occurred across the shallow intra‐trough regions of north‐east Greenland. Collectively these data record for the first time that ice streams were an important glacio‐dynamic feature that drained interior basins of the Late Weichselian Greenland Ice Sheet across the adjacent continental margin, and that the ice sheet was far more extensive in north‐east Greenland during the Last Glacial Maximum than the previous terrestrial–glacial reconstructions showed. Copyright © 2008 John Wiley & Sons, Ltd.     

新疆乌伦古湖冰滑痕特征及其形成机理

新疆乌伦古湖滨湖地区发育一种特殊的冰滑痕。冰滑痕长度可达数十米,宽度可达1~5 m。冰滑痕的形态类似于推土机推过的痕迹,其基本形态由头部、侧翼和滑动面三部分构成。头部发育丘形沙堆,侧翼发育线状沙脊,滑动面总体平整,局部发育冰融沙锥、冰融水滴坑、工具痕、冰融痕、次级滑痕等伴生沉积构造。冰滑痕是湖冰破碎后向岸漂移并在岸上滑移形成的。冰滑痕的形态和规模受湖面风力、湖冰的厚度、湖冰的动能、冰块的大小、冰块底部的光滑程度、湖岸的地形和坡度、湖滩沉积物的粒度和成分构成等多种因素的综合影响。冰滑痕的形成经历了:①湖冰破裂阶段;②浮冰向浅水区运动开始接触湖底阶段;③水下冰滑痕形成阶段;④陆上冰滑痕形成阶段;⑤冰融阶段等5个阶段。冰滑痕主要出现在乌伦古湖的水下滨湖带、冰坝堆积带、湖冰活动带和湖冰影响带等四个微相带。冰滑痕形成后接受湖泊波浪和陆地风的改造并逐渐被埋藏保存到地层记录中。乌伦古湖的冰滑痕是在特殊的气候背景和天气条件下形成的,因而既具有重要的古气候指示意义,又具有重要的古湖泊沉积环境指示意义。